Conventionally, an internal combustion engine for an automobile engine uses a spark plug for igniting an air-fuel mixture by means of spark discharge. In recent years, high output and low fuel consumption have been required of internal combustion engines. To fulfill such requirements, a plasma jet spark plug capable of providing quick propagation of combustion and reliably igniting even a lean air-fuel mixture having a higher ignition-limit air-fuel ratio has been developed.
Such a plasma jet spark plug has a structure in which an insulator formed of ceramics or the like surrounds a spark discharge gap between a center electrode and a ground electrode, thereby forming a small-volume discharge space (cavity). Taking an example of an ignition system of a plasma jet spark plug, in igniting an air-fuel mixture, first, a high voltage is applied between the center electrode and the ground electrode so as to perform spark discharge. By virtue of associated occurrence of dielectric breakdown, current can flow therebetween at a relatively low voltage. Thus, through transition of a discharge state effected by further supply of electric, plasma is generated within the cavity. The generated plasma is ejected through a hole (i.e., orifice), thereby igniting the air-fuel mixture.
In the conventional plasma jet spark plug described in Japanese Patent Application Laid-Open (kokai) No. 2007-287666, an inner surface of an insulator has a stepped portion so as to form a reduced space in the cavity, whereby sufficient ignitability is achieved with about 50-200 mJ of energy. Further, in order to extend a plasma ejected distance and improve ignitability, a plasma jet spark plug according to Japanese Patent Application Laid-Open (kokai) No. 2006-294257 has a cavity of 10 mm3 or less in volume and a ratio of a length to a diameter of the cavity with 2 or more. Further, a distance between a center electrode and a ground electrode is 3 mm or less.
However, since the plasma jet spark plug disclosed in Japanese Patent Application Laid-Open (kokai) No. 2007-287666 includes an insulator having a stepped inner wall therein, channeling (a channel formed due to electric discharge or the like) phenomenon tends to occur and significant deterioration in ignitability is likely to result.
Further, Japanese Patent Application Laid-Open (kokai) No. 2006-294257 discloses a plasma jet spark plug in which a distance between a center electrode and a ground electrode is 3 mm or less, and the center electrode has φ1.5 mm or less. Thus, since the center electrode assumes a long and thin shape, the heat conduction (magnitude of heat decline) of a front end of the center electrode deteriorates, resulting in lowering durability of the center electrode. When the front end of the center electrode has poor heat conduction, the front end of the center electrode is likely to be abraded and eroded, which is prone to cause an oxidization of the front end during the spark discharge at high temperature.
Therefore, the present invention has been achieved in order to solve the above-mentioned problems. An advantage of the invention is a plasma jet spark plug capable of preventing channeling progress and having excellent heat conduction.